In constructing various structures from metals it is important to have the capability of bonding to metal surfaces. This includes bonding metal surfaces to other metal surfaces, as well as bonding non-metal materials to metal surfaces. In many applications it is possible to use simple mechanical bonding mechanisms, such as bolts, screws, or rivets. In other applications, concerns over the added weight of mechanical fasteners make the use of adhesive more viable. Various adhesives are known and commonly used in the art of bonding metals together or bonding non-metal materials to metals. For example, various epoxy-based adhesives are widely used for these applications.
When metals are bonded using an adhesive it is generally important to provide the strongest and most durable bond possible. In the past it was difficult to assure a strong bond when using adhesive. For example, processing conditions during bond fabrication often cause dramatic reductions in bond strength. This is particularly true when bonding to metals such as titanium and titanium alloys. Bonding to titanium materials has presented a special challenge.
Titanium and titanium alloys are considered difficult metals to bond to because of the propensity of titanium surfaces to form a weak hydrated surface layer of titanium oxide. One theory for reduced bond durability of titanium is the reversion of the anatase morphology of TiO.sub.2 to the more stable rutile form. Anatase, the preferred oxide formed by many surface preparation processes, can revert to the rutile form on exposure to warm/moist environments. This conversion is accompanied by a decrease in volume which induces stresses at the adhesive/oxide interface, thereby accelerating joint failure. The morphology of the oxide layer is very important for providing a durable high strength bond. The surface morphology and adhesive bond durability are dependent upon the type of surface treatment received prior to bonding.
Conventional surface treatments of titanium include mechanical, chemical and electrochemical processes. Mechanical treatment includes grit blasting and abrasive processes. Chemical treatments include chlorinated solvents, alkaline cleaners, proprietary caustic etches, nitric-hydrofluoric acid etch, phosphate-fluoride conversion, proprietary nitric acid-chromic acid-fluoride etch (Pasa-Jell), alkaline-peroxide etch and activated chemical oxidation. A VAST (Vought Abrasive Surface Treatment) process combines mechanical and chemical processes by using a high pressure slurry of fine alumina abrasive containing fluorosilicic acid followed by a nitric acid post treatment to prepare the surface. Electrochemical processes include chromic acid anodization, chromic acid-fluoride anodization, alkaline-peroxide anodization and cathodic deposition. These processes include the use of environmentally unfriendly, toxic and hazardous chemicals and are being phased out because of environmental legislation and increasing waste disposal costs.
Accordingly, what is needed in the art are effective and efficient methods of surface preparation and treatment of titanium and titanium alloys, in order to provide stable adhesive bonding to the metal substrate. In that regard it would be a significant advancement in the art to provide methods of titanium surface treatment and preparation which were relatively simple, and which used readily available materials. It would be a related advancement in the art to provide such methods which employed materials that did not present a significant environmental hazard.
It would be a further advancement in the art to provide methods of titanium surface treatment and preparation which provided increased bond strength. It would also be an advancement to provide such which resulted in stable bonds which did not significantly degrade over time. It would be another advancement in the art to provide such methods which resulted in bonds which were unlikely to fail.
Such methods are disclosed and claimed herein.